Control valve system

ABSTRACT

A system having a dual master cylinder for normally transmitting actuating fluid pressure directly to front vehicle brakes and also transmitting another actuating fluid pressure through a proportioning valve to predeterminately reduce the magnitude thereof to rear vehicle brakes, and a control valve responsive to failure of said first named actuating fluid pressure to by-pass said other actuating fluid pressure directly to said rear vehicle brakes and obviate the predetermined reduction in the magnitude thereof.

United States Patent [191 Bueler [54] CONTROL VALVE SYSTEM [75] Inventor: Richard C. Bueler, Glendale, Mo.

[73] Assignee: Wagner Electric Corporation,

Newark, NJ.

[22] Filed: Oct. 17, 1968 [21] Appl. No.: 768,289

Related US. Application Data [62] Division of Ser. No. 626,515, March 28, 1967, Pat.

[52] US. Cl ..303/6 C, 60/54.5 E, 188/151 A, 188/349, 200/82 D, 303/84 A, 340/52 C [51 1" Int. Cl. ..B60t 8/26, B60t 17/22 [58] Field of Search ..303/84, 84 A, 6 C; 60/54.5; 200/82; 340/52; l88/l52.02, 151.11,1'52.11;

[56] References Cited UNITED STATES PATENTS 2,195,214 3/1940 Jacob ..303/84 3,374,322 3/1968 Miller ..200/82 Primary Examiner-Trygve M. Blix Assistant Examiner-John J. McLaughlin Attorney-Joseph E. Papin [5 7] ABSTRACT A system having a dual master cylinder for normally transmitting actuating fluid pressure directly to front vehicle brakes and also transmitting another actuating fluid pressure through a proportioning valve to predeterminately reduce the magnitude thereof to rear vehicle brakes, and a control valve responsive to failure of said first named actuating fluid pressure to by-pass said other actuating fluid pressure directly to said rear vehicle brakes and obviate the predetermined reduction in the magnitude thereof.

9 Claims, 5 Drawing Figures PATENTEDJAN awn 3.708.211 sum 1 BF 2" FIG. 2

44 INVENTOR RICHARD C. BUELER PATENTEDJAN 2m: 3.708.211

SHEU 2 [1F 2 33 3e 35 FIG...5

INVENTOR v RICHARD C. BUELER CONTROL VALVE SYSTEM This application is a division of co-pending application Set. No. 626,515 filed Mar. 28, 1967 now U.S. Pat. No. 3,448,230.

- This invention relates in general to fluid pressure systems and control valves therefor and in particular to those controlling pressure fluid flow between front and rear vehicle brakes.

In the past vehicle brake systems, a dual master cylinder created substantially equal first and second fluid pressures which respectively flowed through a warning valve operative in the event of a predetermined differential between the magnitudes of said first and second fluid pressures to energize a driver warning light through an electrical circuit connected therewith. The first fluid pressure was transmitted directly from the warning valve to actuate front vehicle brakes, and

the second fluid pressure was transmitted from said warning valve through a proportioning valve to actuate rear vehicle brakes, said proportioning valve serving to predeterminately reduce the magnitude of the fluid pressure applied to said rear vehicle brakes to obviate brake locking or vehicle skidding or the like. Of course, the obvious disadvantageous or undesirable feature of such past braking systems was that the proportioning valve was effective to continue to reduce the magnitude of the fluid pressure applied to actuate the rear vehicle brakes when the first fluid pressure for actuating the front vehicle brakes failed.

The principal object of the present invention is to provide a novel fluid pressure system and a novel control valve for use therein which obviates the aforementioned disadvantageous or undesirable features, as well as others, and this and other objects and advantageous features of the present invention will become apparent hereinafter.

Briefly, one aspect of the present invention includes a fluid pressure system having dual fluid pressure generating means therein for respectively actuating a pair of fluid pressure responsive friction devices, means for effecting a ratio between the respective applied fluid pressures, and other means for by-passing said last named means upon failure of one of the applied fluid pressures. Another aspect of the present invention includes a control valve having movable means defining therein a pair of pressure fluid flow passages-and also by-pass passage means for connection with one of said flow passages upon the establishment of a predetermined fluid pressure differential between the respective fluid pressures in said flow passages.

In the drawings which illustrate embodiments of the invention,

FIGS. 1 and 4 are schematic diagrams of brake systems embodying the present invention,

FIGS. 2 and are sectional views showing control valves also embodied in the present invention and for use in the systems of FIGS. 1 and 4 in cross-section, and

FIG. 3 is a fragmentary view taken from FIG. 2 showing an alternative construction of the control valve therein also embodied in the present invention.

, Referring now to FIG. 1, a vehicle fluid pressure or brake system, shown generally at 1, is provided with pressure generating means, such as a dual or tandem master cylinder 2, well known in the art, connected in rear and front brake branches, indicated generally at 3 and 4. The master cylinder 2 is provided with a housing 5 having primary and secondary fluid pressure generating pistons 6, 7 movable therein, said pistons defining a primary pressure generating chamber 8 therebetween and said secondary piston defining with said housing a secondary pressure generating chamber 9. Force transmitting or return springs 8a, 9a are biased between the pistons 6, 7 and between the piston 7 and the end wall of the housing 5 in the chambers 8, 9, respectively. The primary and secondary chambers 8, 9 are respectively connected by compensating ports 10, 11 with divided reservoir chambers 12, 13 in the master cylinder housing 5 and with system branches 4, 3 by outlet ports l4, 15 also in said master cylinder housing. The branch 4 also includes a conduit 16 having one end connected with the outlet port 14 and the other end thereof connecting with a servo motor or wheel cylinder 17 of a vehicle front brake l8, and one of a pair of sets of flow ports of a control or driver warning valve, indicated generally at 19, is serially interposed in the conduit 16. In the branch 3, the other of the pair of sets of flow ports in the control valve 19 is serially interposed in another conduit 20 which has one end connected with the master cylinder outlet port 15 and the other end thereof connected with the input side or port of a proportioning valve or ratio changer 21 of a type generally well known in the art, such as that disclosed in-U.S. Pat. No. 3,232,057 issued to Raymond J. Kersting on Feb. 1, 1966, for instance. The control valve 19 is also provided with a set of delivery ports which are serially interposed in another conduit 22 having one end connected with the output side or port of the proportioning valve 21 and the other end thereof connected with a servo motor or wheel cylinder 23 of a vehicle rear brake 24. To complete the description of the system 1, the proportioning valve 21 is operative in response to fluid pressure applied to the input side thereof in excess of a predetermined value to provide an altered output fluid pressure at the output side thereof in a predetermined constant ratio or a predetermined variable ratio, as desired, with the applied fluid pressure at theinput side thereof, and generally, the magnitude of the output fluid pressure will be less than that of the applied fluid pressure at the input side of said proportioning valve.

The control valve 19 is provided with a housing 25 having a bore 26 therein interposed between aligned counterbores or chambers 27, 28, said counterbores being closed'at their outer ends by plugs or closure members 29 threadedly received therein, and annular shoulders 30, 31 are defined on said housing at the juncture of said bore with said counterbores, respectively. One set of flow ports 32, 33 which receive the conduit 16, as previously mentioned, are oppositely provided in the control valve housing 25 intersecting with the counterbore 27, and the other set of flow ports 34, 35 which receive the conduit 20, as previously mentioned, are oppositely provided in said control valve housing intersecting with the counterbore 28. A pair of annular stepped-recesses 36, 37 are provided in one end of the housing bore 26 connecting with the housing shoulder 31, and delivery ports 38, 39 which receive the conduit 22, as previously mentioned, are also provided in the control valve housing 25 intersecting with the recess 37, said recess 37 defining another chamber in said housing for by-pass connection with'the counterbore 28, as described hereinafter.

An O-ring seal 40 is disposed in the housing recess 36, and an annular spacer member 41 having an axial bore 42 therethrough is provided in the housing counterbore 28 and is normally urged into engagement with the housing shoulder 31 by a spring retainer 43 and spring 44 to prevent displacement of the seal 40 from the recess 36. Another spring retainer 45 in the housing counterbore 27 is biased into engagement with the housing shoulder 30 by another spring 46, said springs 44, 46 having substantially equal and opposed compressive forces. A shiftable piston or switch actuating member 47 is slidably received in the housing bore 26 having opposed ends 48, 49 for abutting engagement with the spring retainers 43, 45, respectively, said opposed ends having substantially equal effective areas A,, A for subjection to fluid pressure in the housing counterbores 27, 28, respectively. The piston 47 is provided with aperipheral seal 50 for sealing engagement with the housing bore 26 adjacent to the leftward end thereof, and another peripheral seal 51 is carried in said piston for sealing engagement with said housing bore adjacent to the recess 37. Spaced peripheral grooves 52, 53 are provided in the piston 47 between the seals 50, 51 and aland or annular abutment surface 54 is defined between said peripheral grooves in sliding engagement with the housing bore 26. The piston 47 extends coaxially through the recesses 36, 37 in sealing engagement with the seal 40 and is provided with a reduced peripheral portion 55 at the rightward end 49 thereof which defines with the spacer member bore 42 and the housing recess 37 a by-pass passage upon actuation of said piston in the leftward direction, as will be discussed in detail hereinafter.

The control valve housing 25 is also provided with a cross-bore and counterbore 56, 57 intersecting the housing bore 26 adjacent to the mid-portion thereof and substantially aligned with the piston land 54. An electrical switch, indicated generally at 58, includes a closure or plug member 59 of suitable insulating material which is threadedly received in the housing counterbore 57, and a metal terminal 60 extends through said plug member. A switch operating member 61 of suitable insulating material has a lower end or follower portion 62 slidably and guidably movable in the housing bore 57, and an electrical contact member 63 is carried on said switch operating member for electrical contact or engagement with a cooperating shoulder or contact 64 defined on the housing 25 at the juncture of the housing bore and counterbore 56, 57. A spring 65 having a current carrying capacity is biased in electrical contact engagement between the switch terminal 60 and the switch contact 63 urging the switch operating member lower end 62 into abutting or positioning engagement with the piston land 54 and urging said contact toward engagement with the housing shoulder 64, said contact being normally spaced from said shoulder when said switch operating member lower end is engaged with said piston land.

Referring now to FIGS. 1 and 2, an electrical driver warningcircuit, indicated generally at 66, is provided to warn the vehicle operator of a failure in the system 1, the control valve housing 25 included in said circuit being grounded by suitable means at 67 wherein the housing shoulder 64 becomes an electrical contact, as mentioned hereinbefore. The external end of the housing terminal 60 is electrically connected by a lead 68 to one side of an electrical driver signal means, shown as a lamp 69, and the other side of said lamp is connected through a vehicle ignition switch, indicated diagrammatically at 70, with a battery 71 by a lead 72, said battery being grounded by suitable means at 73.

In the operation with the component parts of the system 1 and control valve 19 positionedas described above, an operator applied force on the primary piston 6 of the master cylinder 2 concertedly urges the pistons 6, 7 rightwardly to initially close the compensating ports 10, ll isolating the chambers 8, 9 from the reservoirs 12, 13, respectively, and thereafter generate or establish substantially equal fluid pressures in said chambers, respectively. The fluid pressure established in the chamber 8 is transmitted in the branch 4 through the master cylinder outlet port 14, the conduit 16, the flow ports 32, 33 and counterbore 27 of the control valve 19 into the wheel cylinder 17 to effect energization or actuation of the front brake 18. In the branch 3, thefluid pressure generated in the chamber 9 is transmitted through the master cylinder outlet port 15, the conduit 20, the flow ports 34, 35 and counterbore 28 of the control valve l9to the input side of the proportioning valve 21. As previously mentioned, the proportioning valve is operative in response to the generated or established fluid pressure applied to the input side thereof in excess of a predetermined value to provide an output fluid pressure at the output side thereof in ratio with that applied at the input side and having a magnitude proportionally less than that applied at the input side; therefore, the altered or reduced output fluid pressure is delivered or transmitted from the output side of the proportioning valve through the conduit 22, the delivery ports 38, 39 and recess 37 of the control valve 19 into the wheel cylinder 23 to effect energization or actuation of the rear brake 24. It is, of course, obvious that the intensity of the rear brake energization will be proportionally less than that of the front brake due to the proportioning or rationing" effect of the proportioning valve 21. In other words, the

reduced output fluid pressure from the proportioning valve is less than but proportional to the established fluid pressure applied thereto which is, for practical purposes, substantially equal to the established fluid pressure in the branch 3; therefore, the reduced output fluid pressure effecting energization of the rear brake 24 is also proportional to or in ratio with the established fluid pressure'in the branch 4 effecting energization of the front brake 18. It is also obvious that, due to tolerance build-ups of the component parts of the system 1, inherent resistances in said system, or the like, one of the established fluid pressures in the flow chambers 27, 28 of the control valve 19 might fluctuate, spike, or be momentarily slightly greater than the other of the established fluid pressures at the beginning of a braking'application. In this manner, a relatively small differential force created by the momentary and rather small pressure differential between the fluid pressures in the flow chambers 27, 28 acting on the opposed substantially equal effective areas A,, A of the piston 47 isestablished; however, it is contemplated that the compressive forces of the springs 44, 46 are predetermined at a value great enough to ob viate or substantially prevent shifting movement ofthe piston 47 in response to such momentary differential forces.

In the event that the established fluid pressure in the branch 4 fails, due to leaks or the like in said branch or for instance a rupture in the conduit 16, the established fluid pressure in the control valve chamber 28 acting on the effective area A of the piston 47 creates a sustained force to move said piston leftwardly against the compressive force of the spring 46. This leftward movement of the piston 47 in response to the sustained force disengages the peripheral surface of said piston I from the seal 40 and moves at least a portion of the reduced peripheral end portion or by-pass passage 55 of said piston through the seal 40 to connect the chamber 28 in by-pass relation with the housing recess 37 through the by-pass passage defined by the reduced end portion 55 when said piston is in its leftward actuated position. In this manner, the established fluid pressure in the branch 3 is now transmitted from the master cylinder chamber 9 through the outlet port and the conduit to the flow port 34 of the control valve 19 and therefrom through the chamber 28, the spacer member bore 42 and piston by-pass passage 55, the housing recess 37 and delivery, port 39 into the wheel cylinder 23 to effect energization of the rear brakes 24. Therefore, upon the failure of fluid pressure in the branch 4, it is obvious that the established fluid pressure in the branch 3 is transmitted directly to the wheel cylinder 23 effecting full energization of the rear brakes24, and also that the proportioning valve is bypassed or shunted to obviate the altering effect thereof on the. magnitude of the establishedfluid pressure normally delivered to the rear brakes 24. I

,T he leftward movement of the piston 47 upon the failure of fluid pressure in the branch 4, as described above, moves the piston land 54 leftwardly toward a position disengaged from the lower end 62 of the switchoperating member 61, and the compressive force of the switch spring 65 moves said switch operating member downwardly to engage the contact 63 thereof with the housing shoulder 64 and position the switch operating. member lower end 62 within the peripheral groove 53 of the piston member 47. In this manner, ,the engagement of the switch operating member contact 63 with the housing shoulder 64 completes the electrical circuit 66 to light the driver warning lamp-69 by effecting current flow from the battery 71 through the ignition switch 70, the lead 72, said lamp, the; lead 68, the terminal 60, the spring 65, the contact 63 and housing shoulder 64 to the ground at 67. With the driver warning light 69 so lighted to warn the driver of a malfunction in the braking system 1, the abutment between the switch operating member lower end 62 and the side wall of the groove 53 defining the land 54, prevents the return of the piston 47 to its centered or normal position by the compressive force of thespring 46 when the established fluid pressure in the branch 3 is eliminated at the end of a braking application by the removal of the operator applied force.

It is, of course, obvious that the control valve 19 functionsin substantially the same manner to actuate the electrical circuit 66 in theevent that the established fluid pressure in the branch 3 fails wherein the established fluid pressure in the branch 4 acts on the effective area A of the piston 47 to effect rightward movement thereof against the spring 44 and actuate the electrical switch 58, as previously described hereinabove. I

Referring now to FIG. 3, an alternate construction for the control valve 19 wherein the piston 47 is provided with a peripheral seal 80 adjacent to the reduced end portion 55 thereof for sealing engagement with the housing bore 26 between an annular shoulder 81 defined at the juncture of said housing bore and said counterbore 28 and an undercut or recess 82 provided in said housing bore and interconnecting the delivery ports 38, 39. The spring 44 urges the retainer 43 into abutment with the housing shoulder 81. In the event of fluid pressure failure in the branch -4 establishing the sustained force for moving the piston 47 leftwardly against the compressive force of the spring 46, as previously described hereinbefore, the seal 80 is moved leftwardly with thepiston toward a position within the recess 82 and disengaged from the housing bore 26 to open the by-pass passage 55 in said piston between the flow chamber 28 and recess 37 and establish pressure fluid flow from the flow port 34 through said flow chamber, said by-pass passage means, said recess and the delivery port 39 to effect full energization of the rear brake 24 while by-passing the proportioning valve 21;

Referring now to FIG. 4, another vehicle fluid pressure or brake system, indicated generally at 101, is provided with substantially the same component parts and functions in substantially the same manner as the system 1 with the following exceptions.

The system 101 is provided with rearand front brake branches, indicated generally at 102, 103.,The branch 103 is provided with a conduit 104 connected between the outlet port 14 of the master cylinder 2 and the wheel cylinder 17 of the front brake l8, and another conduit 105 is interposed'between the conduit 104 and the flow port 32 of the control valve 19, the control valve port 33 being closed by suitable means, such as a plug or closure member 106. The branch 102 is provided with a conduit 107 connected between the outlet port 15 of the master cylinder 2 and the wheel cylinder 23 of the rear brake 24, and the proportioning valve 21 is serially interposed in said conduit 107. Another conduit 108 has one endintersecting with the conduit 1.07 between the master cylinder outlet port 15 and the proportioning valve 21 and the other end thereof connecting with the flow port 34 of the control valve 19, the control valve ports 35, 38 being closed by suitable means, such as plugs or closure members 109, 110. To complete the description of the system 101, a by-pass conduit 111 has one end intersecting the conduit 107 between the proportioning valve 21 and the wheel cylinder 23 of the rear brake 24 and the other end thereof connecting with the by-pass port 39 of the control valve 19. When the piston 47 of the, control valve 19 is actuated in the leftward direction to open the bypass passage, as described hereinbefore, in response to failure of the established fluid pressure in the branch 103, the established fluid pressure in the branch 102:is delivered to the brake 24-to effect full energization thereof through the conduit 108, the control valve port 34, the counterbore 28, the open by-pass passage into the housing bore recess 37 and therefrom through the is provided through said spacer member substantially coaxial with the housing bore 26, said spacer member bore being intersected by a plurality of cross-passages 204 to communicate said spacer member bore with the ports 34, 35.

A shiftable piston or switch actuating member 205 is slidably received in the housing bore 26 having opposedend portions 206, 207 defining effective areas A,, A and subjected to fluid pressure in the housing counterbores 27, 28, respectively. Peripheral seals 208, 209 are carried in the piston 205 for sealing engagement with the housing bore 26 adjacent to the leftward end thereof and adjacent to the recess 37 therein, and a peripheral groove 210 is provided in said piston having opposed side walls 211, 212 inclined at predetermined angles with the axis of said piston. The piston 205 extends coaxially through the housing recesses 36, 37 in sealing engagement with the seal 40 and is provided with a reduced peripheral portion 213 at the rightward end portion 207 thereof which defines with the spacer member bore 203 and the housing recess 37 a by-pass passage upon actuation of said piston in the leftward direction.

The control valve 201 is also provided with an electrical switch, indicated generally at 214, which includes a conductive closure or plug member 215 threadedly received in the housing counterbore 57, and a metal terminal 216 extends through said plug member and is insulatedtherefrom, said terminal having the exterior end thereof connected with the lead 68 of the circuit 66. A conductive switch member 217 is slidably received in a bore 218 provided in the switch closure member 215, said switch member having a lower end portion 219 extending through the housing cross-bore 56 into following engagement with the peripheral groove 210 of the piston 205 in the'housing bare 26 and having an upper end portion 220 defining a contact for electrical contact or engagement with the interior end of the terminal 216. A spring 221 of predetermined compressive force urges the contact 220 away from the terminal 216 and urges the switch member follower end 219 into positioning engagement with the piston peripheral groove 210. The compressive force of the spring 221 is predetermined at a'value great enough to substantially prevent shifting movement of the piston 205 in conjunction with the force also opposing shifting movement of said piston of the switch member lower end 219 being moved up the inclined peripheral groove surfaces 211 or 212.

In the event that the established fluid pressure in the branch 103 fails, due to leaks or the like in said branch or, for instance, a rupture in the conduits 104 or 105, the established fluid pressure in the control valve chamber 28 acting on the effective area A, of the piston 205 creates a sustained force to move said piston left-wardly against the compressive force of the switch spring 221 urging the switch member 217 into following engagement with the peripheral groove incline surface 212. This left-ward movement of the piston 205 in response to the sustained force disengages and moves at least a portion of the reduced peripheral end portion 213 of said piston through the seal 40 to connect the chamber 28 in by-pass relation with the housing recess 37 through the by-pass passage defined between the reduced end portion 213, the spacer member bore 203 and the housing recess 37 when said piston is in its leftward actuated position. In this manner, the established fluid pressure in the branch 102 is now transmitted from the master cylinder chamber 9 through the outlet port 15 and the conduit 107 and therefrom in bypass or shunt relation with the proportioning valve 21 through the conduit 108, the flow port 34 of the control valve 201, the chamber 28, the spacer member crosspassages and bore 203, 204, the housing recess 37, the

delivery or by-pass port 39 and conduit 111 to the wheel cylinder 23 to effect energization of the, rear brakes 24. Therefore, upon failure of the fluid pressure in the branch 103, it is obvious that the established fluid pressure in the branch 102 is transmitted directly to the wheel cylinder 23 effecting full energization of the rear brakes, and also that the proportioning valve is by-passed or shunted to obviate the altering effect thereof on the magnitude of the established fluid pressure normally to the rear brakes 24.

The leftward movement of the piston 205 upon the failure of fluid pressure in the branch 103, as described above, moves the piston inclined surface 212 leftwardly toward a position driving the switch member 217 upwardly thereon against the compressive force of the spring 221 to engage the contact 220 with the interior end of the terminal 216 completes the electrical circuit 66 to light the driver warning lamp 69 by effecting current flow from the battery 71 through the ignition switch 70, the lead 72, said lamp, the lead 68, terminal 216, the switch member 217 and switch housing 215 to the control valve housing 20 which is grounded at 67. When the branch 103 has been repaired so that fluid pressure can again be established therein, the established fluid pressure in the control valve chamber 27 acts on the effective area A, of the piston 205 creating a force for moving said piston in a rightward direction against the substantially equal and opposing force of the established fluid pressure in control valve chamber 28 acting on area A and at the same time, the established fluid pressure in the chamber 28 is manually reduced to permit the rightward movement of said piston to its original or centered position, as shown in FIG. 5. Although the control valve 201' has been described functioning in the system 101, it is obvious, V

of course, from the foregoing that said control valve will also function in the system 1.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 1

1. In a dual hydraulic brake system having a separate hydraulic pressure supply for each system and each system having at least one pressure responsive brake, means operable generally for modulating the pressure of one of said systems to vary the pressure to the brake thereof, an electrical circuit for energization to warn the vehicle operator of a failure in said one system and the other of'said systems, and a device for by-passing said modulating means and'energizing said circuit, said device comprising a by-pass means subjected to the pressures of said systems for controlling said circuit, said other means beingmovable from a normally centered'position toward opposed translated positions in response to the pressures in one of said one and other systemsuponthe failure of the other of said one and other systems to effect, the energization of said circuit, and said other means including means for controlling said by-pass passage, said included means being mova-' ble with said other means toward at least one of its translated positions to establish flow through said bypass passage to said one system.

2. in a dual fluid pressure brake system having a separate fluid pressure supply for each system and each system including at least one brake, a proportioning valve for one of said systems operable generally for reducing the fluid pressure to the brake of said one system, an electrical circuit for energization to warn the vehicle operator of a failure in said one system and the other of said systems, and a device for by-passing saidproportioning valve and energizing said circuit, said device comprising movable means subjected to the fluid pressures of said systems for comparing the magnitudes thereof," a'by-pass passage in said device and connected in saidone systemin by-pass relation with said proportioning valve, and said movable means including means for controlling said by-pass passage, and other means for, selectively energizing said circuit under preselected conditions, said other means being movable upon movement of said movable means from a normally centered position toward opposed translatedpositions to selectively energize said circuit in response to the fluid pressure in one of said one and other systems upon the failure of the other of said. one and other systems and said included means being movable with said movable means toward at least one of its translated positions to establish pressure fluid flow through said by-pass passage to the brake of said one system. 1

3. In a dual fluid pressure system according to claim 2, comprising signal means connected in said circuit for warning the'vehicl'e operator upon'the failure of said one and other systems, and said device including switch means for engagement with saidother means and con- 5. in a dual fluid pressure brake system according to claim 4, wherein said valve means includes a sealing member engaged with said by-pass passage to interrupt pressure fluid flow therethrough when said piston means is in its centered position and movable to a posi- 'tion disengaged from said by-pass passage and nected in said circuit to normally interrupt energization thereof when said movable means is in its centered position, said'switch means completing said circuit and to energize said signal means upon the movement of said movable means to its translated positions.

, 4. ha dual fluid pressure brake system according to claim ,2, wherein said movable means includes piston meanshaving opposed portions subjected to the fluid pressuresin said systems, and said included means beingoperatively associated with one of said opposed portionsand defining valve means for controlling said by-pass passage, said valve means being movable toward a position providing the pressure fluid flow through said by-pass passage upon the movement of said piston means to its one position, and said other means being on said piston means between said opposed portions.

establishing the pressure fluid flow therethrough upon l the movement of said piston means to its one translated position.

6, in a dual fluid pressure brake system having a pair of system branches and each branch including at least one brake, a separate fluid pressure supply for each branch, a proportioning valve for one of said branches operable generally for reducing the fluid pressure to the brake of said one branch, an electrical circuit for energization to warn the vehicle operator of a failure in said one branch or the other of said branches, and a device for by-passingsaid proportioning valve and energizing said circuit, said device comprising a housing having a pair of chambers therein and connected with said branches, respectively, by-pass passage means in said housing connectingone of said chambers in bypass relation with said proportioning valve in said one branch, piston 'means movable between said chambers from a normally centered position toward opposed translated positions in response to the fluid pressure of one of said one and other branchesup'on the failure of the fluid pressure in the other of said one and other branches, switch means in said housing andconnected in said circuit for controlling energization thereof, means on said piston means for engagement with said switch means to energize said circuit upon the movement of said piston means to its translated positions, and said piston meansincluding valve means for controllingsaid by.-pass passage, said valve means being movable toward a position providing pressure fluid flow through said by-pass passage upon the movement of said piston means to one of its translated positions.

7. In a dualhydraulic brake system having at least one pressure responsive brake in each system, a dual master cylinder having separate pressure supplies in each system and operable generally to transmit pres sure in each system to actuate the brakes thereof, proportioning means in one of said systems operable generally in response to pressure supplied thereto from one of said master cylinder supplies to vary the pressure applied to the brake of said one system under preselected conditions, an electrical driver warningcircuit for energization to warnv the vehicledriver upon the 'failure of the respective pressures in said one system and the other of said systems, and a device connected in each system and in said circuit for by-passing said proportioning means and energizing said circuit under'preselected conditions,- said device comprising a by-pass passage connected in said one system in bypass relation with said proportioning means between said one master cylinder supply and the brake'of said one system, and movable means for controlling said bypass passage and said circuit, said movable means being movable from a normal position toward opposed trans- 8. A vehicle hydraulic split brake system comprising a dual master cylinder having separate pressurizing chambers respectively generating pressures in a first brake pressurizing circuit for energizing a first group of vehicle wheel brake actuators and in a second brake pressurizing circuit for energizing a second group of vehicle wheel brake actuators, a pressure regulator in said first circuit for reducing brake pressure delivered to said first group of vehicle wheel brake actuators from said master cylinder, a brake pressure control sensing brake pressure in said first circuit fluidly intermediate said master cylinder and said regulator and sensing brake pressure in said second circuit and including a pressure regulator bypass piston having a first position connecting said first circuit to said first group of vehicle wheel brake actuators through said regulator, said brake pressure control being responsive to a predetermined sensed brake pressure differential between said sensed brake pressures caused by a pressure loss in said second circuit to place said bypass piston in a second position to bypass said pressure regulator and directly fluid connect said first vehicle wheel brake actuators and said dual master cylinder to deliver full brake pressure through said first circuit without pressure reduction, said brake pressure control having a housing provided with a bore, said bypass piston being reciprocably movable in saidbore in response to said sensed pressurized differential, said bypass piston having an annular space connected to said pressure regulator and to said first group of vehicle wheel brake actuators when said bypass piston-is in said first position, said annular. space being disconnected from said pressure regulator, and said first group of vehicle wheel brake actuators being fluid connected through a working chamber defined by one end of said bypass piston and said housing to said first brake pressurizing circuit intermediate said master cylinder and'said regulator 7 when said-bypass piston is in said second position, and further comprising a pressure failure indicating circuit having a circuit control actuated. by'movement of saidbypass piston from said first to said second position.

9. In a dual hydraulic system having first and second lines normally subject to substantially equal pressures, a sensing unit connected to said first. and second lines to produce a signal responsive to a predetermined pressure differential between said lines, a pressure changing device in one of said lines and valve means operable by said sensing unit responsive to said pressure differential to change the connection of said pressure changing device in said one line, the improvement comprising said sensing unit including a housing, a first member movable in said housing responsive to said predetermined pressure differential and having a neutral position when said pressures are balanced, a second member in said housing, said first and second members being constructed and arranged to provide relative movement between said first and second members by said predetermined pressure differential, cooperating valve means carried by said first and second members and operable by movement ofsaid first member to change the connection of said pressure changing device in said one line, and spring means operable in said neutral position tobias said cooperating valve means into sealing engagement, wherein said first member is axially movagle, one pf said m mbers includes a passage and t e other 0 said mem ers carries a valve means operable to close said passage in one position of said first member, and to open said passage in another position of said first member and wherein said passage is adjacent said first member. 

1. In a dual hydraulic brake system having a separate hydraulic pressure supply for each system and each system having at least one pressure responsive brake, means operable generally for modulating the pressure of one of said systems to vary the pressure to the brake thereof, an electrical circuit for energization to warn the vehicle operator of a failure in said one system and the other of said systems, and a device for bypassing said modulating means and energizing said circuit, said device comprising a by-pass means subjected to the pressures of said systems for controlling said circuit, said other means being movable from a normally centered position toward opposed translated positions in response to the pressures in one of said one and other systems upon the failure of the other of said one and other systems to effect the energization of said circuit, and said other means including means for controlling said by-pass passage, said included means being movable with said other means toward at least one of its translated positions to establish flow through said by-pass passage to said one system.
 2. In a dual fluid pressure brake system having a separate fluid pressure supply for each system and each system including at least one brake, a proportioning valve for one of said systems operable generally for reducing the fluid pressure to the brake of said one system, an electrical circuit for energization to warn the vehicle operator of a failure in said one system and the other of said systems, and a device for by-passing said proportioning valve and energizing said circuit, said device comprising movable means subjected to the fluid pressures of said systems for comparing the magnitudes thereof, a by-pass passage in said device and connected in said one system in by-pass relation with said proportioning valve, and said movable means including means for controlling said by-pass passage, and other means for selectively energizing said circuit under preselected conditions, said other means being movable upon movement of said movable means from a normally centered position toward opposed translated positions to selectively energize said circuit in response to the fluid pressure in one of said one and other systems upon the failure of the other of said one and other systems and said included means being movable with said movable means toward at least one of its translated positions to establish pressure fluid flow through said by-pass passage to the brake of said one system.
 3. In a dual fluid pressure system according to claim 2, comprising signal means connected in said circuit for warning the vehicle operator upon the failure of said one and other systems, and said device including switch means for engagement with said other means and connected in said circuit to normally interrupt energization thereof when said movable means is in its centered position, said switch means completing said circuit and to energize said signal means upon the movement of said movable means to its translated positions.
 4. In a dual fluid pressure brake system according to claim 2, wherein said movable means includes piston means having opposed portions subjected to the fluid pressures in said systems, and said included means being operatively associated with one of said opposed portions and defining valve means for controlling said by-pass passage, said valve means being movable toward a position providing the pressure fluid flow through said by-pass passage upon the movement of said piston means to its one position, and said other means being on said piston means between said opposed portions.
 5. In a dual fluid pressure brake system according to claim 4, wherein said valve means includes a sealing member engaged with said by-pass passage to interrupt pressure fluid flow therethrough when said piston means is in its centered position and movable to a position disengaged from said by-pass passage and establishing the pressure fluid flow therethrough upon the movement of said piston means to its one translated position.
 6. In a dual fluid pressure brake system having a pair of system branches and each branch including at least one brake, a separate fluid pressure supply for each branch, a proportioning valve for one of said branches operable generally for reducing the fluid pressure to the brake of said one branch, an electrical circuit for energization to warn the vehicle operator of a failure in said one branch or the other of said branches, and a device for by-passing said proportioning valve and energizing said circuit, said device comprising a housing having a pair of chambers therein and connected with said branches, respectively, by-pass passage means in said housing connecting one of said chambers in by-pass relation with said proportioning valve in said one branch, piston means movable between said chambers from a normally centered position toward opposed translated positions in response to the fluid pressure of one of said one and other branches upon the failure of the fluid pressure in the other of said one and other branches, switch means in said housing and connected in said circuit for controlling energization thereof, means on said piston means for engagement with said switch means to energize said circuit upon the movement of said piston means to its translated positions, and said piston means including valve means for controlling said by-pass passage, said valve means being movable toward a position providing pressure fluid flow through said by-pass passage upon the movement of said piston means to one of its translated positions.
 7. In a dual hydraulic brake system having at least one pressure responsive brake in each system, a dual master cylinder having separate pressure supplies in each system and operable generally to transmit pressure in each system to actuate the brakes thereof, proportioning means in one of said systems operable generally in response to pressure supplied thereto from one of said master cylinder supplies to vary the pressure applied to the brake of said one system under preselected conditions, an electrical driver warning circuit for energization to warn the vehicle driver upon the failure of the respective pressures in said one system and the other of said systems, and a device connected in each system and in said circuit for by-passing said proportioning means and energizing said circuit under preselected conditions, said device comprising a by-pass passage connected in said one system in by-pass relation with said proportioning means between said one master cylinder supply and the brake of said one system, and movable means for controlling said by-pass passage and said circuit, said movable means being movable from a normal position toward opposed translated positions in response to the failure of fluid pressure in one of said one and other systems to effect the energization of said circuit and in one of the translated positions opening said by-pass passage to establish flow therethrough.
 8. A vehicle hydraulic split brake system comprising a dual master cylinder having separate pressurizing chambers respectively generating pressures in a first brake pressurizing circuit for energizing a first group of vehicle wheel brake actuators and in a second brake pressurizing circuit for energizing a second group of vehicle wheel brake actuators, a pressure regulator in said first circuit for reducing brake pressure delivered to said first group of vehicle wheel brake actuators from said master cylinder, a brake pressure control sensing brake pressure in said first circuit fluidly intermediate saiD master cylinder and said regulator and sensing brake pressure in said second circuit and including a pressure regulator bypass piston having a first position connecting said first circuit to said first group of vehicle wheel brake actuators through said regulator, said brake pressure control being responsive to a predetermined sensed brake pressure differential between said sensed brake pressures caused by a pressure loss in said second circuit to place said bypass piston in a second position to bypass said pressure regulator and directly fluid connect said first vehicle wheel brake actuators and said dual master cylinder to deliver full brake pressure through said first circuit without pressure reduction, said brake pressure control having a housing provided with a bore, said bypass piston being reciprocably movable in said bore in response to said sensed pressurized differential, said bypass piston having an annular space connected to said pressure regulator and to said first group of vehicle wheel brake actuators when said bypass piston is in said first position, said annular space being disconnected from said pressure regulator, and said first group of vehicle wheel brake actuators being fluid connected through a working chamber defined by one end of said bypass piston and said housing to said first brake pressurizing circuit intermediate said master cylinder and said regulator when said bypass piston is in said second position, and further comprising a pressure failure indicating circuit having a circuit control actuated by movement of said bypass piston from said first to said second position.
 9. In a dual hydraulic system having first and second lines normally subject to substantially equal pressures, a sensing unit connected to said first and second lines to produce a signal responsive to a predetermined pressure differential between said lines, a pressure changing device in one of said lines and valve means operable by said sensing unit responsive to said pressure differential to change the connection of said pressure changing device in said one line, the improvement comprising said sensing unit including a housing, a first member movable in said housing responsive to said predetermined pressure differential and having a neutral position when said pressures are balanced, a second member in said housing, said first and second members being constructed and arranged to provide relative movement between said first and second members by said predetermined pressure differential, cooperating valve means carried by said first and second members and operable by movement of said first member to change the connection of said pressure changing device in said one line, and spring means operable in said neutral position to bias said cooperating valve means into sealing engagement, wherein said first member is axially movable, one of said members includes a passage and the other of said members carries a valve means operable to close said passage in one position of said first member, and to open said passage in another position of said first member and wherein said passage is adjacent said first member. 